1. Field of The Invention
The present invention relates to a method and apparatus for detecting myocardial ischemia and more particularly to a system for the non-invasive detection and display of a ventricular wall motion signal for diagnostic purposes.
2. Brief Description of The Prior Art
Coronary artery disease (CAD) is a leading cause of death. Non-invasive techniques for screening patients for coronary artery disease are highly desirable.
At present the most widely used technique for assessing the existence of coronary artery disease is the "stress test" electrocardiogram (ECG). The ECG machine measures electrical signals generated by the heart, at the surface of the patient's body. These ECG signals are displayed to an attending physician for diagnostic use. The ECG waveform displays a characteristic shape, and each cardiac cycle includes P, Q, R, S and T inflection points which correlate to underlying cardiac events. In practice the patient's ECG is taken before, during and after vigorous exercise.
Strenuous physical exercise results in an increase in the patient's heart rate. An elevated heart rate can cause a transient ischemia in the myocardium if a major vessel of the heart is occluded by CAD. The reduction of oxygenated blood to the heart muscle itself affects both the mechanical and electrical properties of the heart muscle. In theory and practice the transient ischemia alters the conduction path of the depolarization wavefront over the myocardium. This abnormal conduction is usually reflected by abnormalities in the displayed ECG waveform. The most typical ECG abnormalities involve the "depression" of the S-T segment of the waveform, and the occurrence of low amplitude, high frequency electrical signals associated with the depolarization of the myocardium. The variability of these signals makes the analysis of the stress ECG highly subjective. Therefore the success of the screening procedure is highly dependant on the physician's skill. Although monitoring the electrical characteristics of the heart to ascertain the existence of coronary arteria disease is wide spread, the signals generated by the heart are ambiguous and difficult to interpret which is undesirable. For these reasons efforts have been directed toward other non invasive and non- electrographic methods of cardiac assessment.
For example, efforts have been directed at monitoring the actual motion of the ventricular wall of the heart to ascertain the existence of coronary artery disease. U.S. Pat. Re. 31,097 reissued Dec. 7, 1982, to Vas et al., shows apparatus capable of detecting the tissue movement of the heart by utilizing an electric field sensing apparatus adjacent to external areas of the body of the person being tested. This non-invasive technique results in a wall motion signal referred to as a "CKG".
In Vas this signal is displayed in real time in connection with a carotid phonocardiogram and a conventional ECG tracing.
The CKG apparatus of the prior art is very sensitive to the placement of the transducer which introduces additional variability in the measurement between pre-stress and post-stress measurements. Additionally, the real time display of the displacement signal does not exclude rate induced variations in the wave form nor are abnormally conducted beats excluded from the display data.
This prior art system exhibits many of the problems which must be addressed to produce a successful system for characterizing and displaying ventricular wall motion.
Like most signals of physiologic origin, there are beat to beat variations of the ventricular motion waveform which do not represent disease states. Other approaches to the direct measurement to ventricular wall motion for assessing myocardial ischemia are reviewed in Ballistocardiography: Past, Present and Future by Goedhard, published in Noninvasive Access to Cardiovascular Dynamics: Experimental and Applied Biblthca Cardiol., no. 37, pp. 27-45 (Karger, Basel 1979).
U.S. Pat. No. 3,695,253 to Vielhauer teaches the use of an accelerometer mounted at a patient's chest for measuring or indicating the blood pressure of a human subject or for detecting exact forces in the cardiovascular system of the patient. Like Vas, Vielhauer displays the output of the accelerometer sensor in real time in conjunction with a ECG waveform derived from the patient for diagnostic purposes.
U.S. Re. No. 27,042 issued Jan. 26, 1971 to Jorgensen, et al., discloses a method and apparatus for automatic screening of cardiac signals. The reference teaches time domain monitoring of low frequency compressional wave information. The preferred range is between 20 and 400 cps.